The statements in this section merely provide background information related to the present disclosure and may or may not constitute prior art.
The majority of passenger car and light and medium duty trucks utilize automatic transmissions having hydraulic fluid control circuits. A hydraulic pump within the transmission provides a flow of pressurized hydraulic fluid which is controlled by valves to actuate various clutches and brakes to select and achieve the desired gear (speed) ratio.
The size of a hydraulic pump, often characterized by the available or generated hydraulic fluid flow rate and pressure, in an automatic transmission can be problematic. On the one hand, it must be large enough to deliver sufficient flow and pressure at low speeds to enable proper operation of the valves, clutches and brakes. A configuration that achieves this will, however, invariably deliver excess flow and pressure at high speeds, flow which is returned through a pressure regulator valve to a sump. Such excess pump capacity at high speeds is obviously inefficient but is an undesirable but essentially inevitable result of sizing the pump for sufficient fluid delivery at low speeds.
One solution to this problem has been the use of two pumps: a first lower volume pump and a second, higher volume pump. This solution is less efficient than a single pump, still requires a pressure regulator valve and also requires control devices which switch between the two pumps in accordance with a predetermined operating schedule.
Variable displacement vane pumps are known in the art. However, such pumps suffer from inefficiencies which conventional, fixed displacement pumps do not exhibit. Thus, a vane pump having a variable displacement capability which exhibits the efficiency of a conventional, fixed displacement vane pump would be desirable. The present invention is so directed.